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Why does spoiler extension during touchdown cause a pitch up moment?
There are reports that incorrect flight technique associated with spoilers extension during touchdown were contributing factors to pitch increase after landing, causing tail strikes in jet transport aircraft.
Why does the spoiler extension causes a pitch up moment in the aircraft after touchdown?

Pitch-up upon spoiler deployment would occur if the spoilers are positioned aft of the center of lift of the wing when in its landing configuration. In this case, the spoiler kills off lift on the aft portion of the wing while the leading side of the wing (in front of the spoilers) is still generating lift. The lift generated by the portion of the wing in front of the spoilers is then not balanced by lift generated by that portion of the wing behind the spoilers, producing a net upwards pitching moment.

Does this come from extending the spoilers before the nose is lowered?

Spoiler deployment on a routine landing using published technique does not cause pitch up on a typical big jet. The moments and forces at play do not cause any alarming nose up tendency.
Generally the published technique recommends that (jet) thrust be brought to idle to coincide with touchdown which means Thrust Levers/throttles be at the idle position a couple of moments before touchdown. The problem arises when the thrust is not at idle during touchdown, and this scenario may be found mostly in handling strong/gusty winds, or as a result of poor piloting technique.
Typical logic for automatic spoiler deployment requires speedbrake lever or 'ground spoilers' armed + thrust at idle + wheels on ground.
In the case where auto deployment does not take place, the spoilers extend once the thrust levers are brought to idle or when the reversers are selected in the case of the system inadvertently not being armed for auto deployment.
Spoiler deployment causes a breakup of the airflow over the wings thus 'dumping lift' and putting weight on the  wheels which is vital for brake effectiveness and traction - and also, without this, the airplane is still very much subject to aerodynamic forces and producing a good amount of lift. A late deployment, even by a second or two gives a considerable nose up pitching moment causing an alarming nose up which, if not actively controlled, can result in  a tail strike or with a wing that is still flying, it can even cause the airplane to skip or bounce, or worse, the airplane can get unintentionally airborne again in an extremely low energy state due to the temporary spike in lift due to increase in the angle of attack.


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How do rotor tangs lock into wheel keyways?
How do the rotor tangs lock into wheel keyways? Are there special keys on the outboard wheel half that allow locking the tangs in place?
Also would be great if anyone could provide me with a image of an aircraft wheel from the inboard side looking outboard.

For what aircraft?

The tangs just slot into mating recesses in the bore of the main wheel so they can float freely laterally as they are squeezed together while transmitting torque from the wheel to the rotors.  Excuse my crude sketch:

Here's a decent cutaway:

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Why is tangential velocity a loss of kinetic energy in a turbine section?
While researching turbine engines I came across a statement

The mass of airflow naturally does not change in the transfer of energy to the rotor system, but the velocity of mass flow being slowed axially is said to account for the energy of flow as the energy of shaft power is increased. In other words, tangential velocity is essentially a loss of kinetic energy to the engine.

Why and how does this the loss occur, and how can this loss be reduced?
I'm not trying to get an in-depth mathematical aspect on that topic as I'm still a beginner with the study of gas turbines, a simpler theoretical non-math explanation would very much be appreciated, thanks!

I am not sure to understand what you mean by Why is the loss a case.

@BambOo I have made the changes in question

Where did you see the axial velocity is slowed through the turbine ? In fact, density of air is reduced during gas expansion which leads to axially accelerate the flow in order to keep mass flow constant. Cross section surface is gradually enlarged in order to confine air speed.

@Salmonbeing00, I read your post a second time and realised that your question is about engine in general and not only the turbine part. My first answer involve turbine module only.

We take axial flow turbines as our example here.
Tangental flow of air (flow that circulates around the engine's central axis) is imparted to the air by viscous drag between the air and the rotating fan and compressor blades. Between those fan discs are stator blades that act to straighten out the tangental flow and help convert the kinetic energy of the axial flow into pressure rise in the compressor stage. Viscous drag losses occur in the stator stages too as they perform this function. These losses dissipate some of the compression work being performed on the air by the compressor as a whole, and that amount of work then cannot contribute to thrust generation in the engine.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Is there an STC for converting the Continental O-470-R to a fuel injected engine?
Is there an STC for converting the Continental O-470-R to a fuel injected engine without making any other changes?

Welcome to the site @ETHEL. You need to add a bit more detail to your question, are you asking about mechanical fuel injection, or electronic fuel injection with a FADEC? When you say 'no other changes' what do you mean?

I’m not aware of an STC to change any engine from carbureted to fuel injected, but there may be an STC to swap the entire engine for a new fuel injected one, and you would trade in the old one or sell it yourself.

Maybe the broader question would be more interesting, is converting an engine from carbureted to fuel injected, something than has ever been approved via an STC?  My hunch is that this would be too much of a change to an engine to be worthwhile, and that swapping the whole thing out is the route you'd have to go. But it'd be interesting to see if there are any counter-examples.

GdD: Mechanical like Bendix and I was thinking about Pponk conversion.

StephenS: Yes could be one option and probably the easiest way.

RalphJ: Didn't think that a conversion like that would be that big. If it is, you are of course right. Have not read the Specs for the IO470 to see the difference, if there are any. I will do that.


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Is it possible to export NMEA data from a Garmin GDL-39 to a custom Android App?
Hi I was wondering if it's possible to receive NMEA data from a Garmin GDL 39 on an Android device (preferred over Bluetooth) as I can't find any information out there on it.

Hi,
The user manual for the GDL 39 shows that the device has Bluetooth, but I can't find a description of how to connect to it outside of the Garmin Pilot app. You might be able to still get NMEA data over serial, but it looks like a lot of work.
What's your use case? Do you have to use a GDL 39 exactly, or do you just want NMEA data into an Android app?

Thanks for the information, that's similar to what I was able to find. I know one of the updates states that it fixed something for NMEA to iOS but no other information. Gdl 39 exactly, the application is setup to read NMEA over Bluetooth and does for a lot of Trimble devices which is an easy configuration of the device but a client is wanting to use our application in a helicopter and has the gdl 39s already.

Well, I think there's a way. Fair warning, it also looks like a fair bit of work, and I haven't tried any of this. Plus, you run the risk that the GDL 39 is going to refuse to connect or do something really weird or otherwise break/invalidate user agreements/warranty.
The user manual for the GDL 39 shows that the GDL 39 has Bluetooth, and this website purports to know the communication protocol and handshake sequence, with code for getting it to work written in Perl. Getting that to work in an Android application shouldn't be impossible, but I haven't looked at it thoroughly and I'm not enough of an Android expert to tell you if it would work off-hand.
You could also go over serial as an option to get NMEA data, which I investigated before I found the website above. I think the Bluetooth option is better, but here's the physical method since I already wrote it up.

You will need a cable like the one here to connect to the GDL over serial. From there, you'll also need a device to take the serial data and get it into the android app over Bluetooth. Here are a few options:

An Arduino or other micro-controller to handle the serial connection to the GDL 39, and then re-broadcast the NMEA data over a Bluetooth connection.

A ioio (pronounced "yo-yo"). Allows you to control a micro-controller with an android application over Bluetooth or via USB. I've used one before, and it's pretty easy to get running, but is a little old. Support still exists for it though.

Your next problem is to actually communicate with the GDL 39 over serial, which will be more complicated.  The GDL 39 appears to have a proprietary handshake protocol over serial, which in theory, should prevent you from using a non-Garmin device to get any data over serial. However, in practice, it may not be hopeless.
This website describes the handshake protocol to negotiate a connection over serial, and data from the GDL 39. I haven't tried to do this, but it looks like it might work. If you can get that to you work, it seems like you'll get a stream of NMEA data, and then you could send it over Bluetooth to an Android application.

Thanks for the reply Tyler.  When seeing the amount of work required we ended up going with a different GPS device that directly exported NMEA data over Bluetooth.

My pleasure. Glad it worked out.


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Are there any certification requirements for steering bypass or lock pins?
Are there any requirements for aircraft bypass and/or lock pins? Do they have to be somehow certified by local or international aviation authorities?

The steering bypass pin on modern airliners serves to isolate the nose wheel from the hydraulics so that movement of the nose wheel will not damage the hydraulic system. What the pin does is usually hold a bypass-valve open, allowing fluid in the actuator which is displaced by the movement of the nose wheel to 'bypass'.
This would be the most simple solution and most manufacturers have adopted this. Exceptions would be aircraft which have an electric nose wheel steering in which case the pin is not used, the pin being deactivated by a switch in the cockpit (ATR72).
In some older aircraft like the DC-8, the drag link (scissor-like link usually seen behind the nose-gear) has a quick disconnect. When disconnected the wheels are only fixed by the central oleo and can rotate thru 360 degrees. A bit of a pain for the push-back crew when reconnecting! On modern aircraft this can be done as well for non-normal operations like in the hangar or aircraft recovery. In normal conditions the actuators limit the angle of the nose wheel.


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How to literally take the wing off a single engine Beechcraft-style airplane?
First off I do not want to do any kind of maintenance. I was driving past an auto junk yard and saw they have half of a low wing, maybe a Beechcraft, airplane and I would like to try and buy the wing for a table or some type of project. I would like to know what kind of tools required to do such a task other than a sawzall!

Taking a wing off can be difficult if you don't know what you are getting into. There are cables, fuel lines, electrical, etc in the wing, one of which can cause a fire. The best thing to do if you really want this is to find an A&P and pay them a couple hundred bucks to take it off.

A normal collection of combination wrenches and sockets, in inches, plus pliers, wire cutters etc, the ole' help along hammer, small bolt cutters for cutting control cable, etc. should do the trick.  Maybe sawhorses to rest the fuselage and wing on.  If the wings are individually removable, they'll be attached with two big bolts or two rows of smaller bolts, just inside the cabin under the seats, and a singe bolt at the back on each side.
The main thing to take care with is there'll be a hydraulic line running out the wing root to the landing gear leg with red hydraulic fluid (similar to ATF fluid) in it for that side's brake, so you should disconnect at each end and drain the brake line first, and there is probably a fuel tank so you need to make sure that is drained and be ready to catch fuel when you disconnect the fuel line connection at the fuselage.   If it's been sitting there for years the fuel is probably all gone but best be ready just in case.  Bring along a fire extinguisher while you're at it.
It's not much different than cutting up a car except the metal is easier to hack at if you want to take the sawzall to it.

Exactly the info I was looking for, Thank you John K and Ron Beyer!! I was a body man for about 9 years so the work itself doesn't scare me I wasn't sure if there were any specialty tools required. Once again thank you both!

@CharlesMGorman as you're not looking to use it as an aircraft part, even if special tools were normally needed you could just take a hacksaw or or cutting tool if need be.


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Can an AFSP training request be renewed when the current one has not yet expired?
My Alien Flight Student Program (AFSP) training request expires in about two months, so the aviation school I'm learning with asked me to just go ahead and create a new training request so there's no wait time between my current request expiration date and whenever the new one would come in.
However, I cannot find an option to create one when the current one is active looks like.
Can you apply for a new training request before the current one expires? If so, how many days before?


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How does a partially covered air intake affect the thrust of a propeller ?
does a partially covered air intake of a ducted propeller significantly affect its thrust?
If so, will it be like 10 or rather 90 percent less thrust ?

Thanks for your answers!
Simon

It certainly would.  You've introduced an inlet restriction as well as a lot of turbulence to the flow.

A full discussion would involve fluid dynamics and might be too broad (although Peter Kampf has a habit of condensing stuff like that nicely) however if would be extremely helpful for you to give an/some examples to narrow the scope.


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What‘s the diameter of the engines of the Airbus E-Fan?
does somebody know the diameter of the E-Fan‘s engines ?
To me the look like around 80cm or what do you think ?
Also, with more and wider blades and 60 kW motors instead of 30 couldn‘t they get much more thrust out of them than 75 kg ?
Thanks for your answers!
Simon

Please be aware that you are asking two different questions, and that the second is not represented in your title, meaning that people will come to read your question to answer the first, not the second.


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Can quadcopters save energy by flying in formation?
If two or more quadcopters are aligned, would the effect of drag due to the lead quadcopter's slipstream lead to measurable energy savings? And if so, how would one determine the optimal distance between the quadcopters?
My thought are that some energy savings are possible, but that there must be some distance between the quadcopters to avoid the high pressure air behind the lead quadcopter interfering with the quadcopter that's behind. How great this distance is depends on the tilt of the lead quadcopter. Since the drag force is proportional with the square of the velocity this means that energy savings should be larger at high velocity, but unfortunately the tilt of the quadcopters will also be higher causing the minimal distance needed between the quadcopters to increase.

My uneducated guess is, no, it would not. Quadcopters are going to produce a lot of turbulence around and behind them (as you have noted), making it more difficult or impractical for those following to fly efficiently. Otherwise, helicopters and airplanes would already be taking advantage of the benefits.

Yes, drag can be reduced by formation flying.

Geese using a low drag "V" formation. The authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001).

Any flying object (plane, bird, helicopter, etc) that creates a down-wash to create lift must also produce an opposite and equal up-wash. A trailing aircraft positioned properly can ride this "wave".

Research aircraft flying a test point (low drag) for the Autonomous Formation Flight project over California's Mojave Desert. From the NASA Dryden Flight Research Center Web site.

The goal of the Autonomous Formation Flight project was to demonstrate 10 percent fuel savings of the trailing aircraft. The project extended the symbiotic relationship of migrating formations of birds. In December 2001, an F/A-18 flying in the wingtip vortex behind another F/A-18 exhibited a 14-percent fuel savings. The project ended in late 2001

But this is all for fixed wing aircraft. I imagine that the dynamics are very different for quadcopters. Perhaps some results for helicopters flying in formation would be more applicable.

A bird is not a fixed wing... It's thrust is closer to a helicopter than a propeller or jet.

Birds and planes use the vortex surfing phenomenon which is not applicable to quadcopters because of the abscence of wings.

Rotating "wings" are affected by vortexes the same as stationary wings. The leading blade is traveling faster than a stationary wing, while the retreating blade is slower. However, It is likely that as much advantage comes from positioning in a low pressure area created by the bow wave, as vortexes; similar to NASCAR drafting.

When quadcopters are flying fast they will need a large tilt to provide the forward thrust. Wouldn't you think that the airflow and high pressure this produces causes interference for the follower?

The fact that a quadcopter has a downwash makes it ideal for an aerodynamic advantage. The down and back beat of a birds wings are very similar to the wake of a quadcopters. for every downwash, there must be an equal upwash.

A computational study by Luo et. al. (Novel Quadrotor Forward-Flight Model Based on Wake Interference) shows a wake interference model wherein each rotor is treated as producing a wake similar to a circular wing of the same size.
Using this model, interference effects with other rotors could be predicted with good agreement to a computational model that included rotor blades as well.  That said, the outcome was more or less a wash in terms of positive or negative effects of interference on the rotors (for what was, effectively, a crosswind in a hovering flight condition).

Body of the second quadcopter will experience less drag. Theoretically, energy savings are there for sure.
However:
- Slipstream must not interfere with the second drone's propellers, because the stability of flight will be severely hindered.
- Since you will want to keep the distance between the leading drone and the following drone as small as possible, additional measures must be taken to prevent collision.


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Are commercial VFR flights legal?
I live in Southern California and was surprised to learn today that a local company is operating very short-haul VFR flights as an alternative to car commuting.
https://www.floatshuttle.com/
I can understand operating out of small airports to keep costs down, but flying VFR for a service like this seems unusual to me. Is this legal?

Related: Can large aircraft go VFR?

It depends on the operator's "opspecs" which are negotiated between them and the FAA. Generally in the US the vast majority of part 121 scheduled airline operations are required to be IFR, but plenty of part 135 charters are permitted to fly VFR. They may be subject to weather minimums higher than the general VFR limits.


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Why doesn't ADS-B use satcoms like ADS-C?
With ADS-C, in the middle of remote areas (like ocean), we can send position report via satellite data link.  So, why ADS-B systems on airplane uses ground based stations (VHF datalink) rather than Geostationary Satellites for data transfer?

Cost, I imagine.

ADS Broadcast and ADS Contract are intended for different purposes. They use the same type of equipment except for the link technology, which is matched to the differing intent.
ADSB is intended to allow self separation between aircraft as well as for informing ground control of position, as a radar would. It is a predetermined set of messages that are broadcast to any nearby aircraft without requiring the interrogation of a ground based radar, as is required for a transponder. VHF is a one to many broadcast technology.
ADSC is intended to provide long distance position tracking, (and weather reporting...) as in transoceanic flight. To conserve bandwidth, there are three types of "contracts" between the aircraft and air traffic control which define the timing of messages sent. The message content in each of the contracts can vary. Only the desired data
is sent on the agreed schedule. Satcom is a one to one messaging technology.


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What is the relation between IAS and TAS at a constant altitude?
I have noticed that when calculating True Air Speed for any given altitude, the faster the Indicated Air Speed, there seems to be a larger increase in TAS?
My thinking was that at a constant altitude the relationship between IAS/TAS would be linear, e.g you double IAS, and TAS would double.. But it seems to increase more? Not sure if I'm missing something here?
For example:
5000ft at 100knts IAS = 114knts TAS
, 5000ft at 110knts IAS = 126knts TAS
Only a couple Knots difference, and it seems to make sense but I can't find any answers on it to confirm it? Would appreciate anybody to shed light on it, thanks!


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Is taxiing a small aircraft on a residential street legal?
I live with the back of my house to a Runway that runs north to south almost perfectly.  I live on the west side of the runway, Across the street from me on the further west most side of the road lives my neighbor who owns runway rights,(his house is completely unconnected to the runway) which from my understanding grants him parking privileges at a designated area on the runway itself. does owning runway parking rights make it legal for him to taxi his plane all 200 yards down the residential road in between my house and his to park his plane in his yard?

at the end of the road there is a opening with access to the runway for people who mostly walk their pets and drive their vehicles to their planes, which is where he is taxing through

If his house is "across the street" then how is it "200 yards... between my house and his"?

What are the laws, rules, and covenants covering the street? Those things can be quite variable. The principal at the Silver Lake, Oregon school used to park his aircraft in the school parking lot, taxiing between there and the airstrip. And I used to taxi a 172 on residential streets there. That was all legal because, at the time, it was not expressly prohibited.

The road is 200 yards long. the road runs between my house and his. @MureyTasroc

thank you @Terry I will look further into my specific area then.

Maybe you could post a location on Google Maps so we can envision this more clearly?

This is a HOA or local traffic law question.  I'm voting to close.

@MichaelHall You want to close a three year old question with positive votes?

@Pilothead, yeah, silly me...  I didn't even notice the date!  It must have had recent activity because it popped up on the top of my list.  Still, I caused someone else to vote for closure as well, and I will stand by my comment that is less about aviation than many we close.

@Pilothead, you are welcome to your opinions, but this is a far cry from asking about making an emergency forced landing on a road.  This question reads exactly like a petty flying neighborhood squabble, with the complainant searching for ammo.

@Pilothead, which is precisely why i think it is a question for the local home owner's association, and not the internet at large.  We are debating opinion here, which never gets anywhere...

It doesn't matter whether or not the road is residential, it depends more on public or private. If the road is a public road, taxiing an aircraft will usually not conform to state motor vehicle regulations without a pilot car and a permit. In rural areas where nobody cares, it doesn't matter. When there are enough people around somebody will object, and then it does matter.
An airpark preserves community access to a runway via residential roads by making them private, and not subject to state or local motor vehicle law. Rules are enforced by conditions on titles to property, or by reference on the title that the property is subject to rules made by an HOA or other organization of the sort. Look at your title.
I think it unlikely you bought into an airpark without knowing it, but if your street signs look like this, taxiing on the streets is probably legal.

It might help to consider what the prop might do if children were playing nearby.

@Robert DiGiovanni: Kids all too frequently get hit by cars, yet AFAIK no one has made driving on residential streets illegal.

It could be worked out locally.


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Does ICAO allow continued flight in RVSM airspace after a transponder failure?
If a civil aircraft's transponder fails in RVSM airspace, can it still be allowed to fly in RVSM according to ICAO?
And could it be allowed to fly in class A airspace, according to ICAO?

Note: I assume bare ICAO requirements in this answer. Most states that have RVSM implementations, as well as operator approvals for RVSM, will have additional stricter rules that will change the answer for specific cases.

There are no requirements in the ICAO Annexes and SARPS that require and aircraft the leave the RVSM airspace if the transponder fails.
Note that RVSM airspace is not necessarily airspace where ATS Surveillance services are offered, so for ATC purposes, a functioning transponder is not strictly necessary in RVSM airspace.
The only other use of the transponder that could prevent flying in RVSM airspace (under bare ICAO regulations) is the Airborne Collision Avoidance System (ACAS/TCAS). There are no requirements to have a functioning ACAS/TCAS in ICAO RVSM requirements, so no transponder is needed in support of such requirement.

Even if a transponder would be necessary to enter RVSM airspace (which is the case in for example EASA regulations), a transponder failure while being in RVSM airspace does (under ICAO rules) does not necessarily mean that the flight has to leave that airspace ASAP.
The following is from ICAO Doc 4444 (PANS ATM).

8.8.3.3    AIRCRAFT TRANSPONDER FAILURE IN AREAS WHERE THE CARRIAGE OF A FUNCTIONING TRANSPONDER IS MANDATORY
8.8.3.3.1    When an aircraft experiencing transponder failure after departure is operating or expected to operate in an area where the
  carriage of a functioning transponder with specified capabilities is
  mandatory, the ATC units concerned should endeavour to provide for
  continuation of the flight to the aerodrome of first intended landing
  in accordance with the flight plan. However, in certain traffic
  situations, either in terminal areas or en-route, continuation of the
  flight may not be possible, particularly when failure is detected
  shortly after take-off. The aircraft may then be required to return to
  the departure aerodrome or to land at the nearest suitable aerodrome
  acceptable to the operator concerned and to ATC.
...

Flight in Class A airspace without transponder is possible under ICAO rules, as long as the airspace is not surveillance airspace. Procedural air traffic control does not require surveillance, and a transponder is therefor not required.

Normally, a transponder failure will mean that you will be required to exit RVSM airspace according to ATC instructions, and either climb or descend out of RVSM without levelling out at any altitude inside RVSM airspace. It is possible to fly in RVSM without a transponder but it requires an altitude reservation not less than 4 or more than 12 hours or more before the flight, and you have to have a reason to ask for it, such as a ferry flight, air ambulance flight, etc.
If it fails in Class A airspace ATC will generally make an effort to let you fly to your destination, but the traffic situation and other factors may prohibit that, in which case you may have to return to your departure airport, land at the closest airport where a repair can be made, or be directed out of Class A as directly as possible for you to then continue your navigation to your destination.

Do you have a reference for the rule about a level reservation 12 hours before a flight? Is that an FAA-specific thing? Bear in mind OP is asking about ICAO recommendation.

http://www.mnps.eu/CH9.htm

That source states that you should request the approval between 4 and 12 (not more than 12) hours before flight, and seems to only apply for MNPS operations (https://www.skybrary.aero/index.php/Minimum_Navigation_Performance_Specifications_(MNPS))

Good catch. Answer edited.

I actually wanna know in which airspace it is mandatory for a civil aircraft to carry a transponder under ICAO regulations?

@HafizKashifAmin Did you search to see if that question has already been answered?

Interestingly, although an functioning autopilot must be equipped, it doesn't have to be engaged. A certain airline once had a requirement for pilots to fly the aircraft manually for a specific distance, while maintaining altitude, in order to meet their certification.


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Why did most WWII Allied piston engines not use fuel injection?
One would think that given the huge push for ever more performance and reliability on piston engines during the heat of WWII would push the designs to their absolute limits, however, most American engines seem to be comfortable with the old carburetor technology which is prone to icing, hiccups and perform poorly under high Gs. Reading the literature of how Pratt & Whitney optimized the Double Wasp to an extreme (to the extent that the crankshaft is taking so much force that it needs a pair of torsional dampers even after forging) is quite jarring when one comes to realize it still uses the good old carbs to suck in fuel, when at the other side fuel injection was adopted eagerly.

Could reliable fuel injection systems have been mass-produced with the technology of the era?  It wasn't until half a century later that they became common in automobile engines.

Those engines didn't use carburetors as in gravity operated float carburetors.  They used "pressure carburetors".  Pressure carbs are mechanical fuel injection systems that inject the fuel from a single point at the start of the manifold instead of injecting fuel at the intake port via a distribution system.  Not quite as theoretically efficient as point fuel injection, but it has all the other advantages including automatic mixture control and no G sensitivity, as well as being less sensitive to damage by not having small high pressure fuel lines running to each cylinder.
On cars in recent years you had a similar system called "throttle body fuel injection", more or less an electronically controlled pressure carb.

so the so-called pressure carb is essentially before-supercharger single port injection? that makes sense

@meatball I think fuel is injected after any air compressor, not before.

@John K thanks for another fine answer.  This one ties with the question about carburetors, fuel richness, and altitude currently active on this site.  Injecting into the intake manifold makes a lot of sense in that it will beneficially cool the compressed air from the turbocharger and feed pre-mixed charge right into the cylinders.  Turbos also work wonders for power output, as well as eliminate the possibility of carb icing.


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How should I answer this question about measuring airspeed using dynamic pressure?
I have come across a question in the PPL Principle of Flight material which I can't understand how to answer:

Give your opinion on the statement "Measuring dynamic pressure is the
  way to determine the indicated airspeed of an aircraft".

Welcome to aviation.SE. You say that you don't understand how to answer that. The question looks like it is coming from a test or similar, so I would expect that you have some material on which you studied to prepare to such a test. In order to answer you, could you let us know what is unclear to you in the material you studied on?

@Federico I think i might have to explain how dynamic pressure works to show IAS at the ASI through the Pitot tube. Then i guess my question would be how does the pitot tube help show IAS and also of the static port is involved.

Welcome to aviation.SE! I edited your question to make it a little clearer but if I got it wrong please edit again or roll back my changes. It looks like you might have translated the question from another language; it might be useful to know which country you're in and see the question in the original language. If you're new to StackExchange, the [tour] might be helpful.

You're confused! Great! Confusion is just an opportunity to learn.
In the spirit of education, I won't answer your question, but I will give you some things to consider.

"Give your argument on the statement" - This usually means you're being asked to support or refute whatever comes next. It will be up to you to decide which is appropriate.
Measuring dynamic pressure - What is dynamic pressure? Are there other kinds of pressure? Is there something else besides pressure that could be measured instead?
is - Is it or is it not?
the way to - the only way? Are there other ways?
determine the indicated airspeed - Is that what is determined? Is anything else determined? Is indicated airspeed the proper thing to determine or do you need something else?
of an aircraft. - Is that correct?

If you can answer all of those, you should have no problem putting them together into a solution. If you're having problems with a specific portion, let us know what's unclear.

I think you missed the most important question of it – what is indicated speed in the first place.

This is a slightly more unusual essay question as they are usually multiple choice.
First define dynamic pressure.  As its name implies, dynamic pressure is wind pressure as the result of movement of the aircraft.  Pressure is measured in units of force (lbf) per area (square inch).  How do we measure indicated airspeed with dynamic pressure?
V squared is proportional to dynamic pressure.  V = square root of Pdyn
Other types of Pressure?  Static pressure, which is dependent on altitude and weather conditions.
Other ways to measure indicated airspeed?  How about a tiny windmill attached to a DC generator and measure voltage vs speed.  Another way would be to take a pressure transducer (like they have on pocket scales)  and place it on a retractable holder (like a nose wheel) to provide a backup airspeed reading in an emergency.

How do you propose to measure indicated airspeed using GPS?

You would have to add in headwind or subtract tailwind.  Might not be as safe.

Let me phrase it differently (it was a rhetorical question): I think they will and should fail you on a PPL test if you mention GPS in the context of indicated airspeed...

Absolutely agree.  Appreciate the edit.


text:
Spanwise flow and wingtip vortices in forward-swept wings
One of the benefits of forward-swept wing is that spanwise flow direction is from tip to root. However, in this answer an X-29 has quite significant sweep angle, at which drawbacks such as aeroelasticity effects dramatically increase.
What happens to the wingtip wake on a forward-swept wing?
Here are some points I'd like to touch:

Does sweeping wings forwarg completely eliminate it, and if it
depends on sweep angle?
How to estimate sweep angle, at which the
spanwise flow starts to flow to the root?
Considering a tampered wing with only trailing edges swept forward, is it enough to get wing characteristics (primarily spanwise flow) like (or close to) forward-swept one?

There is no magical switch that gets flipped once you cross a specific sweep angle. Things are gradually changing with the cosine of the local sweep angle. Now let's look at your questions one by one:

What happens to the wingtip wake on a forward-swept wing?

Nothing special. It is part of the full wake system and behaves like in any other aircraft of comparable aspect ratio and lift distribution over span. For high sweep angles and high lift, the wing torsion will increase local loading towards the tips, making the tip vortex relatively stronger.

Does sweeping wings forward completely eliminate it, and if it depends on sweep angle?

Why should it? No, the tip wake of the wing with more forward sweep will be stronger for comparable wings with different sweep, for aeroelastic reasons as explained above.

How to estimate sweep angle, at which the spanwise flow starts to flow to the root?

See here for the reasons for spanwise flow. As soon as forward sweep results in a lateral local speed component, spanwise flow will happen.

Considering a tapered wing with only trailing edges swept forward, is it enough to get wing characteristics (primarily spanwise flow) like (or close to) forward-swept one?

With a straight leading edge, flow will not exhibit any sweep effect. With a large taper ratio, forward sweep can still become significant towards the trailing edge, but most flow acceleration happens around the nose, so overall sweep effects will still be small.

Peter, thank you very much for answer and links! But will the tapered wing in question benefit from sweeping trailing edge? For example in airleron efficiency near stall speed?


text:
Why was the Sukhoi Su-47 Berkut never adopted for service?
This is a rather long question, since I try to cover the historical background which might not be widely known. If familiar with it, you can skip to the "highlighted sections".

As part of the programme equivalent to the U.S. Advanced Tactical Fighter (ATF), aimed at producing a 5th generation multi-role fighter aircraft, Russia (then USSR) produced (like the U.S. programme) 2 competing designs: one from Mikoyan and the other from Sukhoi. These were the canard/delta Wing 1.42/1.44 from Mikoyan and the FSW (Forward Swept Wing) Su-47 from Sukhoi.
Initially running parallel to the U.S. programme, due to mounting financial difficulties and the eventual collapse of the USSR, the 2 prototypes were ready more than 5 years later than their respective US counterparts (The 1.42/1.44 prototype was completed at 1994-5 but performed its first flight in 2000 while the Su-47 was completed in 1992-3(?) and flew in 1997).
Even before the collapse of the USSR, though, the Soviet Air Force had chosen the Mikoyan design as its next 5th-gen fighter.
Yet, after USSR's collapse, Sukhoi was able to finance its own project and have it ready earlier, while also running an R&D programme and further evaluating/improving the design. By the time the Mikoyan MFI performed its first flight, the Su-47 was already taking part in air shows while serving as a testbed for various technologies (many of them finding their way to PAK FA).

This raises a number of questions, all relating to why the Su-47 was not chosen, neither before the USSR collapsed, and secondly why it was not chosen in the 90s by Russia as it was clearly a design in a more mature stage.

Was the basic design premise-that of a FSW fighter aircraft-flawed?
Were the problems facing this configuration, relating mainly to wing durability, still insurmountable?

To be precise, a FSW design offers superior manoeuvrability, especially in low speeds, retaining an AoA (Angle of Attack) undreamed of by a "conventional" fighter, is essentially spin-proof, it has a better lift/drag ratio, lower stalling speed while it can help with low observability characteristics (Stealth) as radar beams impacting the wings are reflected to the fuselage , ideally one with a comprehensive RAM (radiation absorbent material) covering.
But a FSW design has a major drawback-the loads that accumulate on the wings during manoeuvring, are much greater than in a conventional wing, leading easier to deformation and structural failure. The Su-47 tackled this problem by making extensive use of composite materials for the wings which offer greater durability at less weight.

If the FSW design was viable, was it a matter of economics?

After the USSR collapsed and Russia embarked on economic "reforms" its defence budget shrank dramatically for a decade and new procurement was very difficult. But after 1999-2000, gradually funding increased and when the need for a 5th gen aircraft became again apparent, the PAK FA (Sukhoi T-50) project began, around 2005.
But given that a design was already ready, namely that of Su-47, why not save all the expenses (PAK FA development is in the range of 10 billion US$) while gaining in terms of time?
I usually do not like to speculate on "What if?" historical scenarios but had the Su-47 been chosen, say in 2003-4, by now RuAF could have fielded a fleet of 100+ airframes (assuming initial production starting at about 2008-9).

I do not have inside knowledge, but think it is easy to explain why the Su-47 did not progress beyond the prototype stage:

Sukhoi enjoyed great export success with the Su-27 and its derivatives in the 1990s, and this helped to advance work on the Su-47. Mikoyan was in less favorable financial shape during this time, which explains the delay in getting the 1.44 ready.
There was no credible threat that required Russia to procure a modern fighter design during the last two decades. The existing fleet was simply good enough.
The forward-swept configuration is optimised for close-range air combat, while the cost of modern combat aircraft dictates that they need to fulfill multiple roles. Note that the PAK-FA (Su-57) will replace both the MiG-29 and the Su-27 and their derivatives, so even Russia will not support two frontline fighters in the future.
Much has been learned in the meantime about stealth, so the Su-47 was already outdated by the turn of the century.

I have to admit that when I first saw the Su-47, I was scratching my head and wondered what the Sukhoi engineers were thinking when they proposed this design. But I had similar reactions when the Boeing X-32 was unveiled …

Now it seems the engineers at Sukhoi put what they learnt on the Berkut to good practice when designing the SR-10 two-seat trainer (ainonline.com). The reason for the forward sweep is excellent handling at high angle of attack – however, this bleeds energy quickly and can only be used in few corner cases in air combat. But it certainly helps for pilot training if the possible angle of attack regime is large.

AFAIK Berkut was never intended to be more than a technology demonstrator and research aircraft, similar more to the Rockwell X-31 than the Boeing X-32 which was designed as a prototype for a possible production aircraft (it just never got ordered because for mostly political reasons the LM X-35 was chosen instead as the basis for the F-35).

Thank you very much for your answer! Upvoted but I will wait for a definite (if possible) confirmation on whether the whole concept of a FSW fighter was considered flawed by the-notoriously unwilling to embrace futuristic designs-Soviet High Command, thus leading to rejection or other reasons were at play-particularly later on.
I understand that Sukhoi  was reluctant to participate in the 5th gen competition believing that its Su-27 was the fighter for the 90s and early 00s, so was this meant never to be accepted? But why go ahead then?..Still many questions..
Ahh, the X-32.. hehehe

@jwenting It was the direct product of the Sukhoi entry to the USSR 5th gen fighter competition. Perhaps it was closer to the final design than the 1.44 Mig was. Notice the presence of internal weapon bays for example-something quite telling..But it certainly did end up being just that-an experimental aircraft.

That Sukhoi SR-10 looks like a prize you’d win out of a cereal box!

The Su-47 was not a fifth generation fighter. It is an advanced fourth generation.
Third generation fighters had invested their entire design into a certain set of attributes, most notably high speed and a powerful radar. The idea was that they would rush in, fire their missiles, and run away. Against a slower aircraft with a less powerful radar and shorter range missiles, they would be largely invulnerable.
Unfortunately, plans last until they meet the enemy. In this case they met them over Vietnam, where they quickly learned that getting rid of all other performance niches was a very bad idea. Although the MiG-21 was by no means a very responsive aircraft, it still managed to outperform many US designs. Worse, given the weapons of the era, notably the AIM-7's relatively short range, meant that most engagements would end up in or near visual combat anyway.
The result were the fourth generation designs, which still mounted a powerful radar, but were now designed to be much more maneuverable as well. They had mission statements like "not a pound for air to ground", meaning the aircraft should be as light as possible in order to improve its maneuverability, and damn any other mission. The result were designs like the F-15, F-16 and Su-27, with considerable amount of wing area and powerful engines intended to push out the performance envelope.
Technology changes. Through the 1980s we saw the introduction of the AIM-120, which reliably pushed engagement range out well beyond visual range, and stealth concepts to reduce detection range. Later, we saw the introduction of GaAs TR modules that could be used to build a AESA radar that fit in a fighter, constantly shifting and spreading across frequencies so other aircraft didn't even know it was there.
And thus opened a new performance niche, the fifth generation. Such designs retain all the maneuverability of the fourth, but combine that with a radar and stealth that means they will always see the enemy before the enemy sees them. Ideally, combat between an F-22 and any other aircraft should result in the enemy only realizing they're being attacked when the AIM-120's seeker goes active seconds before impact (likely not even long enough for the RWR to notice).
In this respect, the fifth generation works something like the third. However, because of the range of the missiles of the 1960s, the "advantage range" was on the order of a few miles. With the reduction in detection range via stealth and the increase due to new missiles, now it is so large that the F-22 has huge volumes of space where it can fly around completely invisible to the enemy, and because it knows where they are, it can maintain this invisibility by choosing its location carefully.
Now let's compare that with the Su-47. Here are the design goals for that aircraft:

higher lift-to-drag ratio
better agility in dogfight situations
higher range at subsonic speed
improved stall resistance and anti-spin characteristics
improved stability at high angles of attack
a lower minimum flight speed
a shorter take-off and landing distance

As you can see, almost all of these are improvements on the fourth generation concepts. There's nothing in there about reducing detection range or improving its radar's stealthiness. This is also a big honking design, which a fifth generation design will see at some significant portion of 100 km, place themselves in an advantageous position, and shoot them down. Large numbers of such aircraft could deny this by spreading themselves out and netting their radars, but the cost of these machines is so high that was simply not a possibility.
So basically the world passed the Su-47 by. It is simply not a competitive design.

A LPI radar still needs to use a simple mode like CW to illuminate the target for missile lock. Opening the bay, the stealth fighter itself will also be detected upon launch. So stealth is a tool for shooting first, but missile launch and the missile itself will be just as detectable.

The Su-47 is indeed a 4.5 gen design, but every nation that has introduced new fighters post-Cold-War has launched a 4.5 gen design. Rafale, Typhoon, F-18E. And every one keeps producing 4 gen designs, since there's a huge demand for them; they didn't vanish once US-exclusive 5 gen appeared on the horizon. So there's clearly more factors, such as finances and market forces, that have to be considered.

@Therac - that is not true, CW illumination is not always needed with modern missiles. The missile is fed with initial target coordinates and is fed updates through the midcourse via datalink. It activates its own radar during the terminal phase. Semi-active modes are still possible and will be used if available, but are not required and, AFAIK, not used with LPI.

Yes, LOAL has been a thing for a while, and of course IR missiles don't use a radar at all. But Pk is still maximized with conventional lock on. LOAL isn't stealth aircraft specific and the option to trade hit rate for lower RWR detectability is not either. Stealth is a powerful tool, but it works as part of overall EW effort to reduce enemy knowledge, and it didn't cause a mass cancellation of all non-stealth designs.

The Su-47 was started as an advanced multi-role fighter, and first was designated S-37 (do not confuse for Su-37).
Later it was selected as an advanced carrier-based fighter, renamed to Su-27KM, which is Корабельный Модифицированный (Korabelny Modifitsirovanny, Modified Carrier-based), and then Su-47 (when it turned to be just an experimental platform, I suppose).
Due to financing shortages only 4 instances were built, all at the expense of design bureau.
Its forward-swept wing design along with wide use of composite materials was selected in order to achieve better handling at low speeds and heights.
The reason for it never adopted for service I see further modifications of Su-27(K), as well as lack of aircraft carriers in Russia. Also, development of Su-37 with thrust vector control engine made possible to achieve supermaneuverability at low speeds for Su-27. Still, many technological achievements made during the development were later used in PAK-FA.
(Also, PAK-FA is not going to replace Mig-29s, since later are short-to-medium range interceptors, while Su-27 is a multirole fighter with almost twice dry weight, range and payload).

Among other factors: timing
When we take a look at certain dates:
Su-27: first flight 1977, introduction 1985
Su-47: first flight 1997
PAK FA: first flight 2010
we see that the first flight of the Su-47 coincides with the end-of-90s recession if Russia, see GDP of Russia since 1989.
So we may suspect, that there simply wasn't the financial backing available from official Russia to introduce a new generation of fighter jet.
In the light of economic difficulties it was the prudent way for the Russians to stick to the tried and tested Su-27 family.
The fact, that Sukhoi funded development out of its own pocket may also serve as an indicator that the Russian military never intended to introduce the type.

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text:
Do large aircraft accumulate water in their tanks?
Do large aircraft accumulate water in their tanks?
If so, how is it drained? How often is it drained? At what rate per hour does it accumulate? If not what was done to circumvent the problem?

AFAIK all aircrafts accumulate water in their tanks. Since you ask for large aircraft acccording to wikipeadia:
FAA definition of large aircraft

The US Federal Aviation Administration defines a large aircraft as any
  aircraft with a certificated maximum takeoff weight of more than
  12,500 lb (5,700 kg)

EASA definition of large aircraft

The European Aviation Safety Agency (EASA) defines a large aircraft as
  either "an aeroplane with a maximum take-off mass of more than 5,700
  kilograms (12,600 pounds) or a multi-engined helicopter."

Here is Airbus A340 draining process with explanations on youtube

Here you can find EASA regulations about draining https://www.easa.europa.eu/download/etso/ETSO-C76b_CS-ETSO_11.pdf
Here is FAA regulations about water draining
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC20-125.pdf
On FAA AC 20-125 p.7 states

WATER REMOVAL/PREVENTION. Water is best removed from jet fuel by
  passing the through approved filtration/separation equipment. A
  two-hour settling period and the use of floating suction to drain fuel
  from the top of a storage tank are also recommended to help prevent
  water from being introduced into a fuel system. Water will readily
  settle-out of aviation gasoline; therefore, filters/separators are not
  required for water removal, however, they are necessary for removal of
  solids. The best means to minimize the amount of water entering a fuel
  system is the inspection and proper maintenance of equipment and the
  training of ground and flight personnel.

Also FAA AC 20-125, Water In Aviation Fuels, para 7 c suggests that it may be necessary to gently rock the wings of some aircraft while draining the sump in order to completely drain all the water.

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text:
How is the required motor power calculated for a quadcopter?
I've been trying to find the required motor power for a quadcopter. I first calculated drag power and then I made a linear trend line for motor power against motor weight. I made the motor power 1.5 times the drag power and then used the trend line to find its weight but this seems to give numbers that are much too small. What am I doing wrong and how can I fix this?
What I did was take the altitude, the temperature, estimated weight including payloads, rotor radius and chord, and the speed to solve for drag using the drag equation. An example is alt: 400 ft, temperature: 65 F, estimated weight: 3 pounds, rotor radius: 6 inches, rotor chord: 1 inch, and speed: 20 kts. This would give a drag power of 16 watts and this is so small that the motor weight is negative according to the trendline.

If you can post an example of what you did (including the numbers you used), it would enable us to better help your problem.

Probably answered in How to calculate the required motor power and rotor size with respect to the weight of quadcopter?

Would I just add the power needed to overcome drag to the amount needed to overcome weight?

I think the assumption is to use twice the power required to compensate for weight. Half the power is in excess at sea level, and used to provide for drag, wind, temperature, density, etc, and horizontal and vertical accelerations. I'm not an expert. Drag here seems difficult to evaluate anyway. Indeed this total power has to be split by the number of motors, and the efficiency of the motor / propeller must be taken into consideration.


text:
How do people suffering from hypoxia not realize they aren't getting enough oxygen?
I've seen reports of flight passengers/crew suffering hypoxia, and many times without knowing it.
My question is, how could you not be aware of it? For example, at above 10000ft, if your body is suffering from a lack of oxygen, doesn't it make you breathe faster and harder? (which in turn, makes you aware that something is wrong in the air you're breathing)
Second question: if you're dealing with air containing half as much oxygen, is it not enough (sustainable) to inhale (faster) twice as much air when breathing (without oxygen mask of course) ?

welcome to aviation.SE. for the first question, to which reports are you referring to? for the second question, I'm not sure we can answer, maybe people at biology.se could be better suited.

Air Crash Investigation (i've watched a few :) )

One of the first things to go is your ability to reason.  I've watched aircrew in decompression test chambers and one of the common side effects is that they appear drunk - they are not able to recognise the symptoms.  This is also common to other forms of gas imbalances such as nitrogen narcosis in divers.

On a very related note, Smarter Every Day on Youtube just (today) released a video showing the effects of hypoxia and why people require training before they realize it is happening to them. https://youtu.be/kUfF2MTnqAw

I once hiked up to 10300 feet. When I arrived at the summit, I felt a little euphoric and felt almost not tired. So it felt good. When I was back home, that made me think how I would reacted at higher peaks...

You may read about hypoxia in diving which is very well documented. When there is half the partial pressure of oxygen, you cannot just breath twice frequently. The problem is oxygen pressure, not oxygen quantity. That's why divers use gas compounds, rather than pure oxygen (which would be lighter and take less volume), to manage the partial pressure. Also remember that the heart would have to work quicker too, as well as blood pressure would have to increase, oxygen separation would need to be more efficient, etc.

Your brain needs oxygen to realize anything, including the fact that it's not getting enough oxygen.  In short, biology makes it very hard for a person to realize that they need more oxygen.  The brain's not equipped with a low-oxygen-sensor, it just stops working.  Fairly rapidly, too.

like this one https://en.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash

People lose consciousness very quickly in a low-pressure situation. Moreover, their clarity and decision making is compromised very quickly (within seconds). This is why pilots are trained to test O2 flow and put their masks on very quickly, and before anything else in a depressurisation incident (before even descending).

The key point here is that your body does not measure blood oxygen levels. Instead, the urge to breathe is caused by a surplus of carbon dioxide in your blood. At sea level, this is fine: the only way to get so much carbon dioxide in your blood is if you used up all the oxygen.
At higher altitudes, the pressure is lower, and consequently the oxygen partial pressure. Since the rate of diffusion in your lungs is a function of pressure, this means that oxygen will diffuse less readily into your blood. At the same time, carbon dioxide diffuses more readily out of your blood due to the low pressure. This reduces your urge to breathe; you do not feel 'out of breath'. See this Wikipedia page.
At even higher altitudes, the pressure is actually so low that breathing will cause oxygen to diffuse out of your blood; you can breathe as much as you want but the partial pressure of oxygen in your blood will never be higher than the partial oxygen pressure in the air. This is why oxygen masks work (even if they're not airtight): they replace all air near your nose and mouth with oxygen, so that the partial pressure of oxygen equals the total atmospheric pressure rather than the usual ~21%. See this and this Wikipedia page, or for example this YouTube video.
Somewhat related: at sea level, you can also run out of oxygen before you feel the urge to breathe. You might think that breathing deeply before having to hold your breath for a long time increases the amount of oxygen in your blood, increasing your endurance. This is only partially true: it also artificially reduces your carbon dioxide levels, reducing your urge to breathe. This way, you can black out before feeling out of breath, which can be dangerous under water!

Your answer is quite good and can reach excellency by adding a link to a study or any other reference to confirming your answer, allowing us to read mode about this specific subject.

@ManuH Better like this?

Seems like a major "design flaw". Imagine determining an engine is out of fuel only by measuring a drop in the amount of exhaust!

@ToddWilcox I guess it's because the relative differences in CO2 levels are large since the absolute concentration is low (high sensor sensitivity). But that's my interpretation as an engineer, perhaps there's a better biological explanation.

I was really being humorous. On the serious side, I'm thinking and as far as I can tell, all of our sensations are in response to a positive stimulus. I'm not aware of a sensation that produces nerve impulses when there is a lack of stimulation. Now it could be that oxygen sensing could have developed like sight or hearing where there is a steady stream of impulses which indicate the concentration of O2, but that's pretty complex for something that normally shouldn't need to be sensed. Sensing CO2 above a threshold is more in line with most of our sense systems.

@ToddWilcox Ah that makes more sense. Thanks!

@ToddWilcox: For lack of stimuli resulting in nerve impulse, this is the case when something constant (a noise, a blinking) suddenly disappears (silence, obscurity). This is the change that is detected.

@mins But silence causes the absence of nerve impulses being sent along the otical nerves, not the addition of new nerve impulses. It only seems like a new stimulus because it's so rare that there is not a steady stream of impulses coming from the ears. So a lack of positive stimulation does not cause nerve impulses in the case of sound or vision.

That's why when breathing helium from baloons (or other helium source) you should make sure to breathe some air in-between helium breathes... it's not really funny to see someone with helium-pitched voice suddenly faint and fall to the ground (I saw that one happen once...)

@sanchises: The biological reason is rather straightforward. O2 doesn't dissolve in blood. It's carried by the red blood cells. This makes it hard to sense the concentraion. CO2 on the other hand dissolves, and subsequently forms H2CO3 (carbonic acid). As this is a weak acid, it causes a drop in the blood pH. This directly affects the part of your brain which regulates breathing.

@ToddWilcox: Stimuli are "preprocessed" by certain areas of the brain, constant stimuli are blocked from going further down in the brain processing mechanism. On the other side, for changes these areas generate a new stimulus that is distributed to other brain areas for actual use. That explains why you don't (and can't) see an existing motionless point in certain conditions, but see a ghost point after a bright area is removed from view. See Troxler's fading and other illusions.

Minor quibble:  Anything that raises the acidity (lowers the pH) of the blood will cause an increase in respiration rate:  large doses of aspirin, Vitamin C, and of course carbon dioxide...

No, lack of oxygen, in an of itself does not trigger negative physiological responses; quite the opposite.  Most people who are affected by hypoxia have a general feeling of elation or euphoria and think everything is fine.  The body will, however, respond strongly to high levels of carbon dioxide in the blood and lungs but not low levels of oxygen; this is why you feel a strong need to begin breathing again after holding your breath.
Below is a graphic demonstration of what happens to a pilot in a pressurized aircraft should it suffer rapid decompression and the pilot does not access a pressure breathing/on demand oxygen supply very quickly.  The person feels drunk or euphoric; they believe nothing is wrong with them and that they are able to fly the airplane and operate the onboard systems just fine.  The casual observer, however, sees the same person's cognitive and motor abilities degrading to the point that they cannot even accomplish basic tasks like play with a child's toy for instance or do simple math.  And this condition gets worse and worse, all without the affected person even realizing what's wrong, until unconsciousness, coma and death ensue.  The man in the video below would have died, had it not been for the tech in the chamber with him who forced the oxygen mask back onto his face.

It's amazing how quickly he recovers once he has the mask on.

Similar video by Smarter Every Day.

As I understand it, all mammals do have a physiological response to hypoxic conditions, it's just that humans aren't consciously aware of it outside of some mild light-headedness.  But behind the scenes, your cells are well aware of the lack of oxygen and are going into a panic mode.  The hormone "Hypoxia Inducible Factor" is responsible for regulating your response to hypoxia and when you do eventually die it's because your cells have decided to give up (i.e. cell death occurs through senescence and apoptosis rather than necrosis).

As someone who has experienced hypoxia numerous times, the simple answer is that as you climb at 500-1000fpm into thinner air, the effects are very subtle, and, having compared them with other people, very personal.
For me, at about 86% O2 saturation, measured on a pulse oximeter, the first thing I start to feel is a very slight decline of mental function, such as lack of attention span and difficulty with words. the next effect at about 82% is a headache, and below 80% my mental capacity goes down. I have never had blue lips or nails, or euphoria, as the aeromedical factors in the pilot training tell me to look out for, nor have I blacked out down at 74%.
When I fly in my unpressurized mooney with built-in oxygen, I have the pulse ox on, and I tune the O2 valve to it at every altitude. But I have gotten good enough at recognizing the symptoms for myself, that I can tune my O2 valve by feel.

Wow. Never realised that the effects can be so different from person to person. I guess I should have expected it but I didn't expect it to the extent you're describing.

The 'breathe faster and harder' is a cinema trope.
One of the first things to go with hypoxia is judgement. Do you feel good? You cannot trust your assessment.
As an impecunious glider pilot (not able to afford oxygen gear), the drill my syndicate stick to is this. You don't go above 13000ft. If you're above 10000ft, and you yawn, that's it, you descend to sub 10000, regardless of how good the wave is, regardless of how you feel.

Apart from the points raised in other answers about breathing response being triggered by carbon dioxide in the blood, and the feeling of euphoria when hypoxic, there is also the fact that low blood oxygen impairs brain function. So not only do you feel giddy, but your ability to perform mental tasks degrades - you don't just feel fine, but you can't think straight to process the hazard.
For what it's worth: movies and TV probably have a lot to do with the misconception about human response to hypoxia. It's pretty standard for "the air is getting thin / running out of oxygen" to be portrayed by characters breathing more heavily. Whether or not it started out as an error which has never been corrected, it likely persists in part because it comes across in a more obvious and dramatic way on screen. And so the misconception likely persists in the general population because of how it is depicted on screen.

A simple explanation is that, if you reach the point when the Oxygen in blood is not high enough, then your alertness is reduced, and you may faint, and even die. This is why warnings are to put the oxygen mask yourself, before helping the one in the seat next to yours.
There are devices at a very low price, that measure Oxygen in Blood and Pulse Rate just by putting your finger inside it. 'Pulse-oxymetry' is the name, although it's a medical device, not intended for aviation use.
If you begin to feeling your breath is racing, then you may be too close to losing consciousness for any additional steps to be taken.


text:
What is this no-livery, fully white plane parked at Mumbai airport?

At the center of this picture, there is a white plane with a blank livery.
To me, this plane looks like an Airbus A310. First I thought it was an A330, but then this plane doesn't have winglets, and it looks too "compressed" (I couldn't get a better word) to be an A330.
If it is actually an A310, that's very rare at Mumbai airport.
It is parked in the international cargo area, near the FedEx 777 and National Airlines 747.
I already did Google Image search, and that said it was an A330-300, but I doubt that.
Cropped pic of just the plane:

More than the plane model, I'm interested in the operator and from where it came.

After doing some digging around on FlightRadar24 with the replay function, I was able to find out that this plane in the picture is an Airbus A300-600, which was converted to a freighter in 2005.
It currently has the registration "ER-BUS" and came to Mumbai from Al Ain International Airport in UAE, being operated by "Fly Pro", a cargo company registered in Moldova.
It's 34 years old and has no livery.

(Source)

Nice research!  It is literally a white-box airframe.

Nice research, well done!

I'm expecting Nicolas Cage to inspect cargo

"ER-BUS" ^^, I see what they did there

Some additional info from Planespotters: delivered new to EgyptAir in 1990, converted to cargo in 2005, flew with EgyptAir Cargo until 2019, taken over by Fly Pro in 2023. Fly Pro also flies three 747s, but only owns one of them, and that one is also all-white.

It is very clearly an Airbus A300 to me.
Although the A300 and A310 are similar, there are recognisable differences.

The wingtip are A300 style, not A310.
I count 4 flap track fairings underneath the wing outside of the engine. That is consistent with the A300; the A310 features only 3 flap track fairings on that wing section.

I haven't been able to find out which A300 it is yet.

This is really quite common in cargo carriers, especially those that might rent aircraft for short periods of time - it's often not worth painting a livery.
Some of the fleet for AMX AIr appear completely white and the same for many of Smartlynx Airlines

So you are saying it's either a 767 or an A321?

There's no way that the plane is either of those. Unless you've got a clear shot of the registration number it is going to be extremely difficult to identify an airline, if not impossible. It probably is an A300, though.

@IsraelO.Emmanuel yeah it's gonna be hard to identify the airline, but I think the make and model should be possible to discern


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Why did the Boeing 787 switch back to separate IRS systems?
The Boeing 737 uses two independent IRS systems (see also Why are there two IRS switches on the 737NG?):

(source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators))
and the Boeing 747 uses three independent IRS systems:

(source: 747-400 FCOMv2 (11.10.9 - Flight Management, Navigation - Controls and Indicators))
For the Boeing 777, they switched to a single ADIRU:

(source: 777 FCOMv2 (11.10.10 - Flight Management, Navigation - Controls and Indicators))
The reasons are explained in this answer to the question How many Inertial Reference Systems are on a commercial aircraft?:

Fast forward to the Boeing 777, the pilots only have one such unit: the ADIRU, or Air Data Inertial Reference Unit. The ADIRU by itself is a fault tolerance unit - within this unit, there are multiple gyroscopes and accelerometers. In the past, when multiple redundant units are provided to the pilots, there were instances where the pilots had trouble identifying the malfunctioning unit or switched to the wrong unit, and the plane crashed. Now there is only one unit, and the redundancy mechanism is auto-managed.

This makes perfect sense to me and should have improved safety. But I was surprised to find that the Boeing 787, which is in part based on the 777, switched back to two separate systems:

(cropped from source)
Question: Why did they change back? Was there something wrong with the 777 ADIRU?

I don't know, but what an exceptionally well presented and demonstrated question! Love the pictures, they really helped me follow

Seem like the consideration between integrated systems and separated systems. Integrated systems make it simpler to use and separate systems allow better maintenance and requires/allows interaction.

There were also other practical end use issues as well. Having worked in the manufacturing and repair of theses fault tolerant units, one of the benefits was supposed to be the ability to plan maintenance. You could have a gyro or acceleromter voted out and still be able to take off and fly, since the F-T ADIRU had six gyros/accels and the SAARU four gyros/accels. Problem was the airlines weren't really taking advantage of this. They'd wait until you'd have two gyros or accelerometers voted out before and then be unable to take off due to restrictions. Some of the sensors sat voted out for years, with no replacement occurring.
Of course the airlines pointed the blame back the design. The F-T ADIRU was the third most expensive part of the 777 behind each engine. Most of them also did not carry spares for their repair inventory due to the costs of spares. Spare F-T ADIRU a couple years ago were running north of 2 million a piece. They found that the aircraft had less down time with a split system then using the fault tolerant system. Largely because they had ample spares for the split systems due the spare parts costing 1/10th the cost of the fault tolerants.

Interesting info! Welcome to Av.SE - please continue to contribute here.


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Calculating Ground Speed - E6B vs Pythagorean Theorem
I have been trying to create an excel formula for calculating the ground speed of an airplane using the TAS, Course, Wind Dir, and Wind Velocity.
The issue that I am finding is that I can't properly calculate ground speed. Using pythagorean's theorem, you can use both this site: https://www.mathsisfun.com/algebra/vector-calculator.html and this site http://www.csgnetwork.com/e6bcalc.html to calculate your ground speed. This is how my excel sheet works. However, using the E6B(https://www.e6bx.com/e6b/), I get very different results.
For example:
TAS(Tas) = 100
Course(C) = 120
Wind Dir(Wd) = 40
Wind Speed(Ws) = -20 (20 kts coming from 40 degrees(used in E6B), so -20 in the direction of 40 for pythag)
Using Pythagorean theorem, GS = sqrt((Tascos(C) + Wscos(Wd))^2 + (Tascos(C) + Wscos(Wd))^2).
This will yield a GS of 99 kts using a raw vector addition calculator:

It also yields the same result from the http://www.csgnetwork.com/e6bcalc.html site that can calculate ground speed:

Using the E6B however, with the same variables, I get a totally different result (10 kts off):

I guess in my mind there are 3 possibilities, 1) I have made a mistake somewhere or have a misunderstanding of vector addition, 2) The E6B uses more information than I know about, or 3) the E6B isn't 100% accurate.
Please shed some light on this if you understand! Thank you!

You're asking for somebody to debug your program, and that's awfully close to not being on-topic here at Av.SE.  Your best bet, honestly, is to take some problems with known, accurate, solutions, and run them in both of the calculators you have. One should match the accurate solution, and hopefully the other won't.  Then, knowing that "this" answer is wrong, you can start looking to see in what ways it is wrong and what needs to be changed to make it right.

@RalphJ No i am not. I didn't post a single line of code I was using. The sites I posted are open to the public, and the algorithm i wrote up there is just the formula for magnitude given 2 vectors... And the correct answer was about the difference between heading and course, not a debugging of any equation. The question was really : which solution is accurate of these 3 websites where i put in (what I thought) was the same formula.

Course and heading are not the same.  Course is the direction of your path over the ground.  Heading is the direction you are pointed (and the direction you would travel through a still airmass).
In your second site you entered 120 as a heading, not a course.  Correct that and you get the same figures as the final site.

You're doing the same thing in your calculation: treating the angle as a heading and calculating the resulting course.  The simple vector calculation is that the airplane path through the air (TAS@HDG) plus the movement of the air mass (WS@WD) becomes a resultant path over the ground (GS@C).


text:
Were RCS tests performed on the ATF YF-22/YF-23 prototypes?
I know that radar cross section (RCS) tests were performed on geometric models of both the YF-22 and the YF-23 prototypes submitted for the Advanced Tactical Fighter (ATF) competition, but were the same tests performed on the actual YF-22 and YF-23 prototypes when they were in flight?
Both designs had the correct shaping to their outer machine lines in order to minimize radar cross-section, but there is a noticeable difference between those prototypes and, say, the production F-22 in terms of very small details i.e. RAM inlays, putties, tapes around doors, etc.  A similar phenomenon was observed with the X-35 JSF when compared with the F-35 Lightning II development aircraft. That makes me wonder if such an RCS test was part of the ATF competition.

I wonder how useful that would be, since the measurement would be further off from the final product than simulation and ground testing.

Probably not, I would guess. A numerical model would probably more closely predict the F-22's RCS than the pre-production, pre-EMD YF-22/23. I'm fairly certain they tested scaled models on the pole before the YF-22 even flew in 1991(?). And they did later (1999) verify the F-22's RCS with a 'full-scale replica built to production tolerances'. If YF-22 pole testing did happen, Ben Rich might've mentioned it in Skunk Works, but I don't recall.

Also, here's the YF-23 DEM/VAL presentation by Paul Metz and Jim Sandberg. It's been too long since I've watched it to recall whether they touched on pole testing.

Yes. But not in flight. There are no means of accurately plotting the RCS of an airplane in flight; only one direction can be measured, and it won't even be consistent.

Some of the RCS pole models are now on display at the gates to the Pima Air Museum in Tucson AZ.
The production aircraft are painted with radar absorbent coatings and have special treatments around gaps.  There are many other similar subtle details that probably weren't done to 100% for the YF fly-off.  The aircraft were very much prototypes and under development.  Notice how much smaller the 22's vertical tails got between YF and F.  You can expect that a lot of the RCS details had not been fully developed, technology was evolving very rapidly at that time.

Low radar cross section (RCS) was a principal 1986 requirement for the Advanced Tactical Fighter (ATF) selection program that selected the YF-22 and YF-23 prototypes for a fly-off competition.
I am certain that both prototypes, or at least accurate models, were pole-tested during the evaluation period.  Remember, this was in 1986, when computer modeling of RCS was in its very early stages of development.  This testing was probably done at either the Area 51 RCS test range, or Lockheed's Helendale RCS test range.
For more first hand information about the YF-23, see this YouTube video.


text:
What's holding equipment manufacturers from going into multicore, RTOS-based equipment?
What's holding equipment manufacturers from going into multicore, RTOS- based equipment?
Recently, Rockwell Collins certified its display systems based on RTOS:

Lynx Software Technologies, LynxOS-178 RTOS Deployed by Rockwell Collins in Pro Line Fusion Series of Flight Deck Systems, November 27, 2012.

Why are major players not moving into these segments like RTOS, multicore processor? Is it because of certification difficulties?

Many avionics systems from multiple manufacturers use RTOS.  Is there something missing from your question?

I'm in the Avionics industry for over 8 Years now, I never come across RTOS, Multicore projects yet. May be not working for the right company? :)

I think it might be a language confusion.  Are you asking only about multicore?  I guess so since INTEGRITY, VxWorks etc are all RTOS and have been around for years.

It is about the RTOS and Mutlicore. As I said might have been involved in the RTOS based system development/Testing. Leaving RTOS apart, how about Multicore systems?

Sorry, don't know about that.  I'm an ex-avionics engineer, now in software, and my hardware knowledge is out of date.  Last ones I worked on were single core, but only because multicore hadn't been developer then.

On RTOS and Multicire - "http://www.aviationtoday.com/av/issue/feature/Real-Time-Operating-Systems_78865.html#.VebLASWqqko" and on Multicore "http://mil-embedded.com/articles/multicore-safety-certification-circles/" So I was wondering that, somebody from this community had been through :)

it's a lot harder to test SMP systems and there's no real advantage for avionics?

Why can't somebody sight an official hurdles on certification for multicore, if anybody have involved completely in certification process?

I do believe QNX Neutrino RTOS exists in certified equipment, I'll have to dig to find out which. Though the OS does do multicore, I'd have to dig further to find if the certified cpu is indeed multicore.

Outside of large primary and multi-function displays for avionics, the need for multicore cpus running a full blown RTOS is very low. Plus the cost to license a certified OS can be extremely expensive. The certification cost to develop a custom RTOS is even more prohibitive. It is easier (and faster) to write and certify software written for bare metal.

Development costs, complexity and need.
On a system that has to be hard real time multi-core is a pain(think expensive) and most things that need to be hard real time are not that complex. Things that don't need to be hard real time don't need to be RTOS, so you have one system that handles all the hard real time stuff and one that runs Linux and handles the other stuff.

… and even if there were enough hard real-time things to keep several CPUs busy, it is much better from reliability standpoint to run them on completely separate boards, which is exactly what is done: the critical systems are many independent units, each with a separate small task.

AFAIK multicore technology is currently used in some automotive solutions, however usually not as SMP (symmetrical multicore processing) but either AMP (asymmetrical multicore processing), where each core runs its own dedicated tasks, or as lock-step dual cores running the same code for extra reliability.
This article by Freescale engineers offers some reasons for it being introduced in cars:

Automotive SoCs have traditionally been single core, since not much
  computational work or high end applications were targeted on them.
  Automotives were simpler, so were the applications and so were the
  SoCs. As more and more electronics made room in the automotives, the
  complexity of the SoCs kept on increasing. Now the focus is to have
  most of the automotive under electronic control.
High end automotives produced these days provide features like
  electronic stability control (ESC), traction control system (TCS),
  advanced driver assistance systems (ADAS) etc. These features require
  complex SoCs at heart which can collect, process and transfer data at
  a fast rate from multiple peripherals.
No matter at how much high frequency the single core is operating on,
  it will always have performance bottlenecks & challenges while
  performing multiple tasks. Single core running on higher frequency
  consumes more power. This makes the single core architecture unfit for
  ultra low power applications. Dual core based SOC architecture provide
  better tradeoff in performance and power consumption than single core
  based architectures.

Freescale also has a document Embedded Multicore:
An Introduction expanding on advantages and challenges of multicore chips in embedded environments. For example (on interconnect buses):

Ironically, although microprocessors can perform almost instantly the
  sorts of complex complications that decades ago institutions spent
  millions on and built rooms for, the step into multicore processing
  has brought a simple problem to light: Because the total bandwidth
  must be divided among the bus masters,more cores means less bandwidth
  per core.
Also, with increased bus traffic, the risk of collisions increases and
  this lowers bandwidth even further. In short, a bus does not scale
  well above four cores.

While automotive industry has its own safety & certification requirements, I think it's somewhat easier for manufacturers to experiment with multicore solutions on ground, compared with planes which have to fly properly in all situations and have much more potential for damage, so multicore adoption will likely be much slower.
However, they may be introduced in avionics too at some point:

The Multicore for Avionics (MCFA) working group, which includes
  representatives from BAE Systems, BARCO, Boeing, EADS, ELBIT, GE
  Aviation, Hamilton Sunstrand, Honeywell, Rockwell Collins, Thales, and
  Freescale, was established to help commercial avionics companies
  leverage the performance, power, and size advantages of sophisticated
  embedded multicore processors, such as those from Freescale

source
Report from the 2013 WG meeting: http://onboard.thalesgroup.com/successful-multi-core-for-avionics-working-group-meeting-with-authorities/

Multi core systems means at the processor level, there has to be an interconnect bus that shares data. This interconnect brings some level of indeterminism at nano-second levels - what if the data requested is in the cache of core 1 and the process is in core 2? What if contention arises?
Also, when RTOS starts the micro-kernel how will it leverage multiple cores - as separate units? Will it meet stringent safety guidelines if it shares L3 cache but has separate L1 / L2 cache?

We've gotten very good at building cache coherence algorithms: https://en.wikipedia.org/wiki/Cache_coherence
There are alternative mechanisms to program on SMP systems, and it's actually much closer to what existing systems use for called message passing, or the actor model.


text:
How many Harriers crashed at or near MCAS Cherry Point?
When I was stationed at MCAS Cherry Point in the late 1970's, we had a Harrier flip over and crash while practicing at the hover pad. It took off in the direction of our large hangar and thankfully went into the other side (depot maintenance) instead of ours (SOES) where we had a C-9B, T-39's and CH-46 aircraft, all fully fueled. I came back from lunch to find the Harrier in the parking lot at the other side of the hangar, after having plowed through what was left of another Harrier crash, among other things.
I was wondering. How many Harriers were lost in or near MCAS Cherry Point? Does anyone know?

Related

According to List of Harrier Jump Jet family losses on Wikipedia, which appears to be derived from the Aviation Safety Network and is presumably accurate, 25.

There was one that was based there but crashed some distance away so I left that out.

That sounds about right.

The 1 May 1980 crash is the one I was talking about above. The description is not quite correct, the plane did not roll and then flip, but the rest of it, albeit generally described, is accurate. So many good Marines died. :( The only amusing fact about that crash is that the squadron sh*tbird at SOES got himself a medal for running to the other side of our hangar and being the first to fight the fires.


text:
Do U.S. aircraft carriers still include a Marine Corps squadron?
When I was in the 3rd Marine Air Wing back in the early 70s, there was one Marine Corps squadron on each navy aircraft carrier, but on some shows I've seen recently (mostly on the Smithsonian Channel) I haven't noticed any Marine Corp aircraft. This leads me to wonder if they're stilled deployed on the carriers.Edit: I found this picture in the 2018 Marine Aviation Plan
Notice the "VMFA-251" marking on the fuselage. This would make it a Marine Corps aircraft. Just above the picture is this blurb:

The F/A‐18A‐D community continues combat operations for the sixteenth straight year as Hornets support Operation INHERENT RESOLVE: Landbased with SPMAGTF‐CR and shipboard on aircraft carrier deployments as part of our TACAIR Integration (TAI) commitment.

This certainly implies that some Marine Corps aircraft are deployed on some aircraft carriers, but not necessarily all of them.

Does this Wikipedia page answer your question?

@RonBeyer No, it doesn't as it only lists the squadrons and their home base.  You need to find the deployment history for the squadron to find out if it was deployed on a carrier. In a limited search, I was only able to find recent helicopter and Harrier deployments on amphibious assault ships (LHA/LHD), not on CVNs. I didn't see any carrier deployments for the F/A-18.

@Gerry The Sea Knight and Osprey are both flown by the USMC. The Osprey being operated entirely (as of 2018) by the USMC (for carrier operations, the Navy has ordered some but haven't taken delivery yet), so any picture you see of an Osprey landing on a carrier is a USMC carrier deployment.

It was my understanding that the Marine Corps squadrons deployed on aircraft carriers were VMFA squadrons, so they would have been fixed wing, though that's not to say some Marine helicopters weren't.

According to Wikipedia's page on Carrier Air Wings, there are nine active carrier air wings currently active (for eleven aircraft carriers).  Of those nine, CVW-9 includes Marine Corps squadron VMFA-314 operating the F-35C Lightning II, and CVW-17 includes VMFA-323, which operates the F/A-18C Hornet.
So out of nine carrier air wings, two include Marine Corps squadrons.  Not all carrier air wings include Marine Corps squadrons.  I believe other Marine Corps squadrons train on aircraft carriers, but are not permanently associated with air wings like VMFA-312 and VMFA-323 are.


text:
Why does NASA have F/A-18s?
I always remember playing FSX as a kid and using the NASA F/A-18, and never thought much about it.  And then I saw the picture of the Space Shuttle on top of the 747 being escorted by these planes.
Why does NASA have these fighter jets?
What are they usually used for?  Military?  Science?  Advertising NASA?

Who says they're designed for any sort of combat at all, instead of as, say, chase planes?

@cpast well, they are ex-Navy fighters, so they were obviously designed for combat :) ... they just have been demilitarized and are no longer capable.

@egid Yeah, poor word choice on my part (I meant "intended," but wrote "designed").

They are to defend us against aliens, simple.

Obviously a small Cessna isn't going to keep up with the SCA. Still, there are plenty of cheaper planes which could. But remember that NASA is highly political. Want to be that those F-18s are maintained in a state whose senator is quite influential? "Cheap" sometimes is a bad thing.

Battle ready?  I see no missiles.  There is one object on the centerline mount that can't be identified but very likely is a fuel tank--and if it isn't it's some sort of electronics, not a weapon.

@MSalters, it's probably also for public relations.  I mean, they're giving the public a pretty fricking awesome show right now (in the picture).  Had they chosen something like Citation Xs, it wouldn't shout 'Murica nearly as much as F-18s do.

@SpongeBob The use of a fighter isn't for PR. A Citation X is not nearly as good a chase plane as an F-18, because it's far less maneuverable. Chase planes fly in close formation with experimental aircraft, which is a role that military-type planes (they also use T-38s a bunch) are much better at than bizjets.

I believe this is the video that goes with your picture: https://www.youtube.com/watch?v=r6vm_BRK6LU (it's gorgeous) and also https://youtu.be/MhwSbeiRADA?t=55 is an older example from a carrier test flight.

NASA uses them for pilot training and as chase planes for research aircraft.

The two-seat F/A-18 support aircraft are normally used for photo or video chase. They are configured to transmit live video pictures from the air back to Dryden so engineers can visually monitor the mission as it is being flown. This feature greatly enhances flight safety.

There's an article from Armstrong Research Center with some more information, but that pretty much sums it up.
NASA also currently operates (and has operated) many other military aircraft types, either as chase planes or as subjects of research projects. Some examples off the top of my head include F-15s, F-16s, and of course their ubiquitous B-52s.
You say "obviously they are battle ready", but they are not – NASA aircraft are unarmed, and the ones in the photo certainly are carrying no missiles. They may have been fighters in a previous life, but the weaponry has been stripped. "Escort" doesn't mean "fighter escort" in this case!

Now I look like a fool for saying "obviously they are battle ready!"  But what's the point of an escort if it can't kill an attacker?  (Obviously suicide crashing is an option, as after 9/11, but not a good one.)

In this case, escort probably mostly means chase - providing extra eyes on the shuttle and the carrier plane. They're there for support and in the event that there is an emergency.

@SpongeBob "Escort" doesn't always mean "defend against an attacker." In this case, it means something more like "fly alongside to support if there's a problem." I'm pretty sure NASA's level of worry that someone will fly hostile aircraft to bring down their plane is just about zero, and if they were worried they'd call the air force; NASA is a civilian agency, and its only armed employees are its inspector general and its security guards. As for why fighters: F-18s are very agile planes that can easily fly alongside just about any other aircraft, very close to it, without crashing into it.

Also the use of these craft in particular may be due to NASA's extensive work during the development of the F-18 (especially wind tunnel testing).  I suspect the acquisition may be related to that history.

@SpongeBob: In this particular case, the most likely reason is that they watch (and film) the orbiter, looking for any damage so the transporter could make emergency landing if the orbiter was damaged (e.g. in bird strike) and there was risk something could come loose and cause further damage.

NASA also flew SR-71s until 1997 for research: http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-030-DFRC.html

@cpast Also, let's not forget that agility in an escort plane is important when flying alongside research aircraft -- you might need to bug out in a hurry if it goes instable!

They also use T-38s, which are the trainer version of the F-5.

They're mostly chase planes (source: NASA). NASA has four of them, two single-seat and two two-seaters. They provide more eyes on the plane for safety purposes (they are in communication with the pilot), and assist NASA's flight test missions. The two-seaters are also useful for photos and videos of flight tests so engineers have more info.